Are the seedlings indoors? They hate cold nights and cold rain and yellow leaves are a symptom of chilly weather - anything below 55 degrees. If they are planted in the ground try putting plastic glass cups over each seedling at night to keep in the heat. Another reason for yellow leaves is lack of nutrition. Once the first few weeks pass new seedlings have used up the nutrients in their seed storage and need nitrogen. Dilute a complete fertilizer such as Miracle grow to half strength and water your seedlings letting the fertilizer water land on the foliage of your seedlings. Plants take in nutrients fast from foliar feeding. You should see your seedlings perk up in just a few days if lack of fertilizer was the reason for the leaf yellowing. Write back and let us know if this works. Keep growing, Marianne Binetti